hush: add support for special vars in braces

Some people like to use ${?} rather than $?, so make sure we support all
the special single char vars that use this form.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

shell/hush_test/hush-vars/var3.tests

@@ -1,4 +1,4 @@
 # reject invalid vars
 "$THIS_SH" -c 'echo ${1q}'
 "$THIS_SH" -c 'echo ${&}'
-"$THIS_SH" -c 'echo ${$}'
+#"$THIS_SH" -c 'echo ${$}' -- this is valid as it's the same as $$